After having read most of the postings in this thread, I suggest that you attempt to change the focus away from galaxies and focus more on the moon, but not when it's full. A full moon is an acquired taste that very few people manage to acquire. Some of the brighter star clusters can also be rewarding observing choices. And of course, any of the obvious naked eye planets will also be worth a try with your telescope.
8 years old doesn't give you much to work with in the realm of observing galaxies unless you're fortunate enough to live out in the middle of nowhere -- where the Milky Way is an obvious naked eye sight, and where there are no outdoor lights of any kind within sight and no brightly lit cities within 10s of miles. But even then, 8 years old can make galaxy observing a disappointing endeavor -- especially when the child has become accustomed to seeing many of the best photographs that have been taken of such objects.
At your grandson's age, more likely than not he won't be interested in careful, patient observing in order to (barely) see more. More likely, he'll just want to look and instantly see enough to impress him. And even that may not be possible. The bigger problem (than "finding galaxies") is that the child has already acquired some very unrealistic expectations -- and that's most unfortunate.
The moon can be made more interesting by pointing out some of the more prominent features and referring to them by their names. Together, you and grandson can research further information on some of the lunar features that you've both seen with the telescope.
But like Tony mentioned, if the Great Orion Nebula (M42) was a disappointing sight for the child, then galaxies (despite the child's desire to see them) are likely to be even greater disappointments once he's finally able to see them for himself in a telescope's eyepiece.
Anyway, the above is just another opinion, and of course, it might even be an opinion that you and your grandson will eventually prove to be incorrect. We can hope . . .
